1. Field of the invention
The present invention relates to a preservation method of microcapsules for electrophoretic display devices and its applications.
2. Description of the Related Art
An electrophoretic display device displays character data, image data, and the like, by the behavior of electrophoretic particles, for example, when a voltage is applied to a dispersion in which the electrophoretic particles are dispersed in a solvent. For example, if the electrophoretic particles and the solvent are colored with different colors, the color of the electrophoretic particles is observed when the electrophoretic particles are moved to the surface of the solvent by voltage application, and the color of the dispersion is observed when the electrophoretic particles are moved to the bottom of the solvent. If electrodes to which voltage application is made possible by address appointment are provided, different colors for the respective addresses can be displayed, and accordingly, arbitrary character data and image data can be displayed. Moreover, it is possible to rewrite the display data, and at the same time, it is advantageous that the display data can be maintained as it is, even if there is no electric signal.
In recent years, in place of the conventional electrophoretic display devices (e.g., see Japanese Patent Publication No. 50-15115) in which a dispersion of electrophoretic particles are enclosed in spaces between opposed electrode substrates, there have been developed microcapsule type electrophoretic display devices (e.g., see Japanese Patent No. 2551783) with a structure in which microcapsules enclosing a dispersion of electrophoretic particles are arranged between opposed electrode substrates. As compared with the conventional electrophoretic display devices, the microcapsule type electrophoretic display devices are remarkably improved in various performances and functions, such as long-term stability of display, responsiveness, contrast, and display rewritable times.
Electronic equipments using electrophoretic display devices have various sizes, and correspondingly, the electrophoretic display devices also have various sizes. Further, there are many types of substrates needed to undergo the application of microcapsules for electrophoretic display devices. Therefore, it is not a good idea that microcapsules for electrophoretic display devices are preserved in the form of a sheet obtained by applying the microcapsules to a substrate. Accordingly, with respect to the preservation form of microcapsules for electrophoretic display devices, it is desirable to perverse the microcapsules for electrophoretic display devices in the stage before the microcapsules are made into a sheet in terms of utmost avoidance of vainness, efficient productivity, and advantageous resource utilization.
From the above-described viewpoints, when preserving microcapsules for electrophoretic display devices, it is supposed to be possible to preserve the microcapsules for electrophoretic display devices in the form of a synthesized dispersion obtained at the time of preparing the microcapsules for electrophoretic display devices, in the form of a paste obtained by filtering a dispersion of the microcapsules for electrophoretic display devices, or in the form of a coating solution obtained by making the microcapsules for electrophoretic display devices into a coating material.
Conventionally, as a preservation method of microcapsules, which are not microcapsules for electrophoretic display devices but contain an enclosing member, for example, Japanese Patent Laid-Open Publication No. 62-291568 discloses preserving the microcapsules in a preservation solution having an osmotic pressure higher than of the enclosing member by from 3% to 50% and reports that preservation stability is improved no less than 5 times as much as that when preserving the microcapsules in an isotonic solution. However, such a method can be applied only in the case where the enclosing member is a hydrophilic substance, and just like microcapsules for electrophoretic display devices, in the case where a dispersion containing electrophoretic particles dispersed in a hydrophobic organic solvent is enclosed, the above method cannot be employed.